In general, a solid oxide fuel cell (SOFC) comprises a pair of electrodes (anode and cathode) separated by a ceramic, solid-phase electrolyte. To achieve adequate ionic conductivity in such a ceramic electrolyte, the SOFC operates at an elevated temperature, typically in the order of about 1000° C. The material in typical SOFC electrolytes is a fully dense (i.e. non-porous) yttria-stabilized zirconia (YSZ) which is an excellent conductor of negatively charged oxygen (oxide) ions at high temperatures. Typical SOFC anodes are made from a porous nickel/zirconia cermet while typical cathodes are made from magnesium doped lanthanum manganate (LaMnO3), or a strontium doped lanthanum manganate (also known as lanthanum strontium manganate (LSM)). In operation, hydrogen or carbon monoxide (CO) in a fuel stream passing over the anode reacts with oxide ions conducted through the electrolyte to produce water and/or CO2 and electrons. The electrons pass from the anode to outside the fuel cell via an external circuit, through a load on the circuit, and back to the cathode where oxygen from an air stream receives the electrons and is converted into oxide ions which are injected into the electrolyte. The SOFC reactions that occur include:Anode reaction: H2+O═→H2O+2e−CO+O═→CO2+2e−CH4+4O═→2H2O+CO2+8e−Cathode reaction: O2+4e−→2O═
Known SOFC designs include planar and tubular fuel cells. Applicant's own PCT application no. PCT/CA01/00634 discloses a method of producing a tubular fuel cell by electrophoretic deposition (EPD). The fuel cell comprises multiple concentric layers, namely an inner electrode layer, a middle electrolyte layer, and an outer electrode layer. The inner and outer electrodes may suitably be the anode and cathode respectively, and in such case, fuel may be supplied to the anode by passing through the tube, and air may be supplied to the cathode by passing over the outer surface of the tube.
In certain commercial applications, it is desirable to provide a fuel cell system having a relatively high power density, i.e. a fuel cell system that provides a high power-to-volume ratio. Such high power densities may be achieved by assembling multiple tubular fuel cells in close proximity to each other to produce a fuel cell stack. Also, higher power densities can be achieved by increasing the active surface area per unit volume of the system; for example, the active surface area per unit volume can be increased by decreasing the diameter of each tubular fuel cell, thereby increasing the number of fuel cells that can be stacked in a given volume. Such small-diameter fuel cells, especially if made of ceramic or some of its composites, can be fragile and relatively vulnerable to damage when assembled into a tightly packed array. Thin walled elongate ceramic structures tend to be particularly fragile, and may fail when subjected to bending forces or vibrations that exceed the fracture stress of the ceramic.